Optical and electrochemical properties of iron oxide and hydroxide nanofibers synthesized using new template-free hydrothermal method

We report the effect of hydrothermal synthesis conditions on the morphological, optical and electrochemical properties of as-prepared iron oxide (γ-Fe 2 O 3 ) and hydroxide (α-FeOOH) nanostructures. The physico-chemical identification of these Fe-based nanostructures using X-ray diffraction, scanning/transmission electron microscopy, porosity and Raman spectroscopy analyses revealed a temperature-depended phase transformation. A maghemite and goethite iron-based nanostructured formation was observed in nanorod and trigonal nanofiber shape-like morphology with mean diameters ranging from 32 to 50 nm. The textural analysis of the nanofibers confirmed mesoporosity with a specific surface area of ~ 129 m 2 g −1 (in γ-Fe 2 O 3 ) and 23 m 2 g −1 (in α-FeOOH). The electrochemical performance of the iron oxide and hydroxide nanofiber electrodes with and without the addition of activated carbon (AC) was also investigated. The sample electrodes composed of γ-Fe 2 O 3 , γ-Fe 2 O 3 /AC, α-FeOOH and α-FeOOH/AC showed remarkable specific capacities of 164 mAh g −1 , 330 mAh g −1 , 51 mAh g −1 and 69 mAh g −1 at 1 A g −1 gravimetric current. The influence of the phase transformation linked to the synthesis temperature, and the inclusion of an electric double-layer AC material into the nanofibers clearly demonstrates an enhancement in their energy-storage capability. Furthermore, the Fe-based nanofibers exhibited excellent cycling stability with good capacity retention of 73% and 99.8%, respectively, after 2000 cycles at a high 30 A g −1 gravimetric current as well as low resistance obtained by impedance spectroscopy analysis. The implication of the results depicts the potential of adopting these γ-Fe 2 O 3 nanorods as suitable material electrodes in electrochemical energy-storage devices. Graphic abstract